From Wikipedia, the free encyclopedia

A thunderstorm, also known as an electrical storm, a lightning storm, or simply a storm is a form of weather characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere known as thunder.[1] The meteorologically-assigned cloud type associated with the thunderstorm is the cumulonimbus. Thunderstorms are usually accompanied by strong winds, heavy rain and sometimes snow, hail, or no precipitation at all. Those which cause hail to fall are known as hailstorms. Thunderstorms may line up in a series or rainband, known as a squall line. Strong or severe thunderstorms may rotate, known as supercells. While most thunderstorms move with the mean wind flow through the layer of the troposphere in which they occupy, vertical wind shear causes a deviation in their course at a right angle to the wind shear direction. Thunderstorms result from the rapid upward movement of warm, moist air. They can occur inside warm, moist air masses and at fronts. As the warm, moist air moves upward, its cools, condenses, and forms cumulonimbus clouds that can reach heights of 10 km. As the rising air reaches its dew point, water droplets and ice form and begin falling the long distance through the clouds towards Earth's surface. As the droplets fall, they collide with other droplets and become larger. The falling droplets create a downdraft of air that spreads out at Earth's surface and causes strong winds associated with thunderstorms.

Thunderstorms can generally form and develop in any geographic location, perhaps most frequently within areas located at mid-latitude when warm moist air collides with cooler air.[2] Thunderstorms are responsible for the development and formation of many severe weather phenomena. Thunderstorms, and the phenomena that occurs along with it, can produce numerous risks and hazards to populations and landscapes. Damages that result from thunderstorms are mainly inflicted by downburst winds, large hailstones, and flash flooding caused by heavy precipitation. Stronger thunderstorm cells are capable of producing tornadoes and waterspouts.

Life cycle

Warm air has a lower density than cool air, so warm air rises within cooler air,[3] similarly to hot air balloons.[4] Clouds form as relatively warmer air carrying moisture rises within cooler air. As the moist air rises, it cools causing some of the water vapor in the rising packet of air to condense.[5] When the moisture condenses, it releases energy known as latent heat of fusion which allows the rising packet of air to cool less than its surrounding air,[6] continuing the cloud's ascension. If enough instability is present in the atmosphere, this process will continue long enough for cumulonimbus clouds to form, which support lightning and thunder. Meteorological indices such as convective available potential energy and the lifted index can be used to assist in determining upward vertical development of clouds.[7] Generally, thunderstorms require three ingredients to form:

Moisture

An unstable airmass

A lifting force

All thunderstorms, regardless of type, go through three stages: the developing stage, the mature stage, and the dissipation stage.[8] The average thunderstorm has a 24 km (15 mi) diameter. Depending on the conditions present in the atmosphere, these three stages take an average of 30 minutes to go through.[9]

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Developing stage

The first stage of a thunderstorm is the cumulus stage, or developing stage. In this stage, masses of moisture are lifted upwards into the atmosphere. The trigger for this lift can be insolation heating the ground producing thermals, areas where two winds converge forcing air upwards, or where winds blow over terrain of increasing elevation. The moisture rapidly cools into liquid drops of water due to the cooler temperatures at high atitude, which appears as cumulus clouds. As the water vapor condenses into liquid, latent heat is released which warms the air, causing it to become less dense than the surrounding dry air. The air tends to rise in an updraft through the process of convection (hence the term convective precipitation). This creates a low-pressure zone beneath the forming thunderstorm. In a typical thunderstorm, approximately 5×108kg of water vapor are lifted into the Earth's atmosphere.[10]

Mature stage

In the mature stage of a thunderstorm, the warmed air continues to rise until it reaches existing air which is warmer, and the air can rise no further. Often this 'cap' is the tropopause. The air is instead forced to spread out, giving the storm a characteristic anvil shape. The resulting cloud is called cumulonimbus incus. The water droplets coalesce into larger and heavier droplets and freeze to become ice particles. As these fall they melt to become rain. If the updraft is strong enough, the droplets are held aloft long enough to be so large that they do not melt completely and fall as hail. While updrafts are still present, the falling rain creates downdrafts as well. The simultaneous presence of both an updraft and downdrafts marks the mature stage of the storm, and produces Cumulonimbus clouds. During this stage, considerable internal turbulence can occur in the storm system, which sometimes manifests as strong winds, severe lightning, and even tornadoes.[11]

Typically, if there is little wind shear, the storm will rapidly enter the dissipating stage and 'rain itself out',[8] but if there is sufficient change in wind speed and/or direction the downdraft will be separated from the updraft, and the storm may become a supercell, and the mature stage can sustain itself for several hours.[12]

Dissipating stage

In the dissipation stage, the thunderstorm is dominated by the downdraft. If atmospheric conditions do not support super cellular development, this stage occurs rather quickly, approximately 20–30 minutes into the life of the thunderstorm. The downdraft will push down out of the thunderstorm, hit the ground and spread out. This phenomenon is known as a downburst. The cool air carried to the ground by the downdraft cuts off the inflow of the thunderstorm, the updraft disappears and the thunderstorm will dissipate. Thunderstorms in an atmosphere with virtually no vertical wind shear weaken as soon as they send out an outflow boundary in all directions, which then quickly cuts off its inflow of relatively warm, moist air and kills the thunderstorm.[13] The downdraft hitting the ground creates an outflow boundary which can cause downbursts, a potential hazardous condition for aircraft flying through it as a substantial change in wind speed and direction occurs, resulting in decrease of lift of the aircraft. The stronger the outflow boundary is, the stronger the resultant vertical wind shear will become.[14]

Classification

Conditions favorable for thunderstorm types and complexes

There are four main types of thunderstorms: single cell, multicell, squall line (also called multicell line) and supercell. Which type forms depends on the instability and relative wind conditions at different layers of the atmosphere ("wind shear"). Single cell thunderstorms form in environments of low vertical wind shear and last only 20–30 minutes. Organized thunderstorms and thunderstorm clusters/lines can have longer life cycles as they form in environments of significant vertical wind shear, which aids the development of stronger updrafts as well as various forms of severe weather. The supercell is the strongest of the thunderstorms, most commonly associated with large hail, high winds, and tornado formation.

Single cell

This term technically applies to a single thunderstorm with one main updraft. These are the typical summer thunderstorms in many temperate locales. They also occur in the cool unstable air which often follows the passage of a cold front from the sea during winter. Within a cluster of thunderstorms, the term "cell" refers to each separate principal updraft. Thunderstorm cells occasionally form in isolation, as the occurrence of one thunderstorm can develop an outflow boundary which sets up new thunderstorm development. Such storms are rarely severe and are a result of local atmospheric instability; hence the term "air mass thunderstorm". When such storms have a brief period of severe weather associated with them, it is known as a pulse severe storm. Pulse severe storms are poorly organized and occur randomly in time and space, making them difficult to forecast. Single cell thunderstorms normally last 20–30 minutes.[9]

Multicell cluster

This is the most common type of thunderstorm development. Mature thunderstorms are found near the center of the cluster, while dissipating thunderstorms exist on their downwind side. Multicell storms form as clusters of storms but may then evolve into one or more squall lines. While each cell of the cluster may only last 20 minutes, the cluster itself may persist for hours at a time. They often arise from convective updrafts in or near mountain ranges and linear weather boundaries, usually strong cold fronts or troughs of low pressure. These type of storms are stronger than the single cell storm, yet much weaker than the supercell storm. Hazards with the multicell cluster include moderate-sized hail, flash flooding, and weak tornadoes.[9]

Multicell lines

A squall line is an elongated line of severe thunderstorms that can form along and/or ahead of a cold front.[15][16] In the early 20th century, the term was used as a synonym for cold front.[17] The squall line contains heavy precipitation, hail, frequent lightning, strong straight line winds, and possibly tornadoes and waterspouts.[18]Severe weather, in form of strong straight-line winds can be expected in areas where the squall line itself is in the shape of a bow echo, within the portion of the line which bows out the most.[19]Tornadoes can be found along waves within a line echo wave pattern, or LEWP, where mesoscale low pressure areas are present.[20] Some bow echoes which develop within the summer season are known as derechos, and they move quite fast through large sections of territory.[21] On the back edge of the rain shield associated with mature squall lines, a wake low can form, which is a mesoscale low pressure area that forms behind the mesoscale high pressure system normally present under the rain canopy, which are sometimes associated with a heat burst.[22] This kind of storm is also known as "Wind of the Stony Lake" (Traditional Chinese:石湖風 - shi2 hu2 feng1, Simplified Chinese: 石湖风) in southern China.[23]

Supercell

The setting sun illuminates the top of a classic anvil-shaped thunderstorm cloud in eastern Nebraska, United States.

Supercell storms are large, severe quasi-steady-state storms which feature wind speed and direction that vary with height ("wind shear"), separate downdrafts and updrafts (i.e., precipitation is not falling through the updraft) and a strong, rotating updraft (a "mesocyclone"). These storms normally have such powerful updrafts that the top of the cloud (or anvil) can break through the troposphere and reach into the lower levels of the stratosphere and can be 15 miles (24 km) wide. At least 90 percent of this type of thunderstorm bring severe weather.[12] These storms can produce destructive tornadoes, sometimes F3 or higher, extremely large hailstones (4 inches / 10 centimetres diameter), straight-line winds in excess of 80 mph (130 km/h), and flash floods. In fact, most tornadoes occur from this type of thunderstorm.[24] Supercells are the most powerful type of thunderstorm.[9]

Severe thunderstorm

A severe thunderstorm is a term designating a thunderstorm that has reached a predetermined level of severity. Often, this level is determined by the storm being strong enough to inflict wind or hail damage. A storm is generally considered severe if winds reach over 90 kilometres per hour (56 mph), hail is 1 inch (25 mm) in diameter or larger, or if funnel clouds and/or tornadoes are reported.[25][26][27] Though a funnel cloud or tornado indicates the presence of a severe thunderstorm, a tornado warning would then be issued in place of a severe thunderstorm warning. In Canada, a rainfall rate greater than 50 millimetres (2.0 in) in one hour, or 75 millimetres (3.0 in) in three hours is also used to indicate severe thunderstorms.[28] Severe thunderstorms can occur from any type of storm cell. However, multicell, supercell, and squall lines represent the most common forms of thunderstorms which produce severe weather.[12]

Forms of MCS that develop within the tropics use either the Intertropical Convergence Zone or monsoon troughs as a focus for their development, generally within the warm season between spring and fall. More intense systems form over land than over water.[32][33] One exception is that of lake-effect snow bands, which form due to cold air moving across relatively warm bodies of water, and occurs from fall through spring.[34] Polar lows are a second special class of MCS which form at high latitudes during the cold season.[35] Once the parent MCS dies, later thunderstorm development can occur in connection with its remnant mesoscale convective vortex (MCV).[36] Mesoscale convective systems are important to the United States rainfall climatology over the Great Plains since they bring the region about half of their annual warm season rainfall.[37]

Motion

Thunderstorm line viewed in reflectivity (dBZ) on a PPI (NOAA)

The two major ways thunderstorms move are via advection of the wind and propagation along outflow boundaries towards sources of greater heat and moisture. Many thunderstorms move with the mean wind speed through the Earth's troposphere, or the lowest 8 kilometres (5.0 mi) of the Earth's atmosphere. Younger thunderstorms are steered by winds closer to the Earth's surface than more mature thunderstorms as they tend not to be as tall. Organized, long-lived thunderstorm cells and complexes move at a right angle to the direction of the vertical wind shear vector. If the gust front, or leading edge of the outflow boundary, races ahead of the thunderstorm, its motion will accelerate in tandem. This is more of a factor with thunderstorms with heavy precipitation (HP) than with thunderstorms with low precipitation (LP). When thunderstorms merge, which is most likely when numerous thunderstorms exist in proximity to each other, the motion of the stronger thunderstorm normally dictates future motion of the merged cell. The stronger the mean wind, the less likely other processes will be involved in storm motion. On weather radar, storms are tracked by using a prominent feature and tracking it from scan to scan.[12]

Back-building thunderstorm

A back-building thunderstorm is a thunderstorm in which new development takes place on the upwind side (usually the west or southwest side in the Northern Hemisphere), such that the storm seems to remain stationary or propagate in a backward direction. Although the storm often appears to be stationary or even moving upwind on radar, this is actually an illusion. The storm in reality is a multi-cell storm with new, more vigorous, cells being formed on the upwind side replacing older cells which continue to drift downstream.[38] When this happens, catastrophic flooding is possible. In Rapid City, South Dakota, in 1972, an unusual alignment of winds at various levels of the atmosphere combined to produce a continuous, stationary cell which dropped an enormous quantity of rain, resulting in devastating flash flooding.[39] A similar event occurred in Boscastle, England, on 16 August 2004.[40]

Hazards

Each year, many people are killed or seriously injured by severe thunderstorms despite the advance warning. While severe thunderstorms are most common in the spring and summer, they can occur just about any time of year.

Cloud-to-ground lightning

Cloud-to-ground lightning frequently occur within the phenomena of thunderstorms and have numerous hazards towards landscapes and populations. One of the more significant hazards lightning can pose is the wildfires they are capable of igniting.[41] Under a regime of low precipitation (LP) thunderstorms, where little precipitation is present, the lack of rainfall cannot prevent fires from starting when vegetation is dry as lightning produces a concentrated amount of extreme heat.[42] Wildfires can devastate vegetation and the biodiversity of an ecosystem. Wildfires that occur close to urban environments can inflict damages upon infrastructures, buildings, crops, and provide risks to explosions, if should the flames be exposed to gas pipes. Direct damage is caused by lightning strikes occurs on occasion.[43] In areas with a high frequency for cloud-to-ground lightning, like Florida, lightning causes several fatalities per year, most commonly to people working outside.[44]

Precipitation with low potential of hydrogen levels (pH), otherwise known as acid rain, is also a frequent risk produced by lightning. Distilled water, which contains no carbon dioxide, has a neutralpH of 7. Liquids with a pH less than 7 are acidic, and those with a pH greater than 7 are bases. “Clean” or unpolluted rain has a slightly acidic pH of about 5.2, because carbon dioxide and water in the air react together to form carbonic acid, a weak acid (pH 5.6 in distilled water), but unpolluted rain also contains other chemicals.[45]Nitric oxide present during thunderstorm phenomena,[46] caused by the splitting of nitrogen molecules, can result in the production of acid rain, if nitric oxide forms compounds with the water molecules in precipitation, thus creating acid rain. Acid rain can damage infrastructures containing calcite or other solid chemical compounds containing carbon. In ecosystems, acid rain can dissolve plant tissues of vegetations and increase acidification process in bodies of water and in soil, resulting in deaths of marine and terrestrial organisms.[47]

Large hailstones

Any thunderstorm which produces hail that reaches the ground is known as a hailstorm.[48] Thunderclouds that are capable of producing hailstones are often seen, obtaining green coloration. Hail is more common along mountain ranges because mountains force horizontal winds upwards (known as orographic lifting), thereby intensifying the updrafts within thunderstorms and making hail more likely.[49] One of the more common regions for large hail is across the mountainous northern India, which reported one of the highest hail-related death tolls on record in 1888.[50]China also experiences significant hailstorms.[51] Across Europe, Croatia experiences frequent occurrences of hail.[52]

In North America, hail is most common in the area where Colorado, Nebraska, and Wyoming meet, known as "Hail Alley."[53] Hail in this region occurs between the months of March and October during the afternoon and evening hours, with the bulk of the occurrences from May through September. Cheyenne, Wyoming is North America's most hail-prone city with an average of nine to ten hailstorms per season.[54]

Hail can cause serious damage, notably to automobiles, aircraft, skylights, glass-roofed structures, livestock, and most commonly, farmers' crops.[54] Hail is one of the most significant thunderstorm hazards to aircrafts. When hail stones exceed 0.5 inches (13 mm) in diameter, planes can be seriously damaged within seconds.[55] The hailstones accumulating on the ground can also be hazardous to landing aircraft. Wheat, corn, soybeans, and tobacco are the most sensitive crops to hail damage.[50] Hail is one of Canada's most costly hazards.[56] Rarely, have massive hailstones have been known to cause concussions or fatal head trauma. Hailstorms have been the cause of costly and deadly events throughout history. One of the earliest recorded incidents occurred around the 9th century in Roopkund, Uttarakhand, India.[57] The largest hailstone in terms of maximum circumference and length ever recorded in the United States fell in 2003 in Aurora, Nebraska, USA.[58]

Tornadoes and waterspouts

A tornado is a violent, dangerous, rotating column of air which is in contact with both the surface of the earth and a cumulonimbus cloud (otherwise known as a thundercloud) or, in rare cases, the base of a cumulus cloud. Tornadoes come in many sizes but are typically in the form of a visible condensation funnel, whose narrow end touches the earth and is often encircled by a cloud of debris and dust.[59] Most tornadoes have wind speeds between 40 and 110 mph (64 and 180 km/h), are approximately 250 feet (76 m) across, and travel a few miles (several kilometers) before dissipating. Some attain wind speeds of more than 300 mph (480 km/h), stretch more than a mile (1.6 km) across, and stay on the ground for dozens of miles (more than 100 km).[60][61][62]

The Fujita scale and the Enhanced Fujita Scale rate tornadoes by damage caused. An EF0 tornado, the weakest category, damages trees but not substantial structures. An EF5 tornado, the strongest category, rips buildings off their foundations and can deform large skyscrapers. The similar TORRO scale ranges from a T0 for extremely weak tornadoes to T11 for the most powerful known tornadoes.[63]Dopplerradar data, photogrammetry, and ground swirl patterns (cycloidal marks) may also be analyzed to determine intensity and award a rating.[64]

Formation of numerous waterspouts in the Great Lakes region.

Waterspouts have similar characteristics as tornadoes, characterized by a spiraling funnel-shaped wind current that form over bodies of water, connecting to large Cumlonimbus clouds. Waterspouts are generally classified as forms of tornadoes, or more specifically, non-supercelled tornadoes that develop over large bodies of water.[65] These spiralling columns of air are frequently developed within tropical areas close to the equator, but are less common within areas of high latitude.[66]

Flash flood

Flash flooding is the process where a landscape, most notably urban environments, is subjected to rapid floods.[67] Flash flooding can frequently occur in slow-moving thunderstorms and are usually caused by the heavy liquid precipitation that accompanies it. Flash floods are most common in dense populated urban environments, where less plants and bodies of water are presented to absorb and contain the extra water. Flash flooding can be hazardous to small infrastructure, such as bridges, and weakly-constructed buildings. Plants and crops in agricultural areas can be destroyed and devastated by the force of raging water. Automobiles parked within experiencing areas can also be displaced. Soil erosion can occur as well, exposing risks of landslide phenomena. Like all forms of flooding phenomenon, flash flooding can also spread and produce waterborne and insect-borne diseases cause by microorganisms. Flash flooding can also be caused by extensive rainfall released by hurricanes and other tropical storms, as well as the sudden thawing effect of ice dams.[68][69] Human activities can also cause flash floods to occur. When dams, constructed for hydro-electricity, have failed, large quantities of water can be released and can destroy everything within its path.[69]

Downburst

Downburst winds can produce numerous hazards to landscapes experiencing thunderstorms. Downburst winds can generally be extremely powerful, and are often mistaken for wind speeds produced by tornadoes,[70] due to the concentrated amount of force exerted by their straight-horizontal characteristic. Downburst winds can be hazardous to unstable, incomplete, or weakly-constructed infrastructures and buildings. Agricultural crops, and other plants in nearby environments can be uprooted and damaged. Airplanes and other aviation transportations can be exposed to risks of crashing during takeoffs and landing periods.[8][70] Automobiles can be displaced by the force exerted by downburst winds. Downburst winds are usually formed in areas when high pressure air systems of downdrafts begin to sink and displace the air masses below it, due to their higher density. When these downdrafts reach the surface, they spread out and turn into the destructive straight-horizontal winds.[8]

Frequent occurrences

Thunderstorms occur throughout the world, even in the polar regions, with the greatest frequency in tropicalrainforest areas, where they may occur nearly daily. Kampala and Tororo in Uganda have each been mentioned as the most thunderous places on Earth,[71] an accolade which has also been bestowed upon Bogor on Java, Indonesia or Singapore. Thunderstorms are associated with the various monsoon seasons around the globe, and they populate the rainbands of tropical cyclones.[72] In temperate regions, they are most frequent in spring and summer, although they can occur along or ahead of cold fronts at any time of year.[73] They may also occur within a cooler air mass following the passage of a cold front over a relatively warmer body of water. Thunderstorms are rare in polar regions because of cold surface temperatures.

Types of lightning

Lightning is an electrical discharge that occurs in a thunderstorm. It can be seen in the form of a bright streak (or bolt) from the sky. Lightning occurs when an electrical charge is built up within a cloud, due to static electricity generated by supercooledwater droplets colliding with ice crystals near the freezing level. When a large enough charge is built up, a large discharge will occur and can be seen as lightning.

The temperature of a lightning bolt can be five times hotter than the surface of the sun.[75] Although the lightning is extremely hot, the duration is short and 90% of strike victims survive. Contrary to the popular idea that lightning does not strike twice in the same spot, some people have been struck by lightning over three times, and skyscrapers like the Empire State Building have been struck numerous times in the same storm.[76] The loud bang that is heard is the super heated air around the lightning bolt expanding at the speed of sound. Because sound travels much more slowly than light the flash is seen before the bang, although both occur at the same moment.

There are several types of lightning:

In-cloud lightning is the most common. It is lightning within a cloud and is sometimes called intra-cloud or sheet lightning.

Cloud to ground lightning is when a bolt of lightning from a cloud strikes the ground. This form poses the greatest threat to life and property.

Ground to cloud lightning is when a lightning bolt is induced from the ground to the cloud.

Cloud to cloud lightning is rarely seen and is when a bolt of lightning arcs from one cloud to another.

Ball lightning is extremely rare and has several hypothesized explanations. It is seen in the form of a 15 to 50 centimeter radius ball.[77]

Cloud to air lightning is when lightning from a cloud hits air of a different charge.[78]

Dry lightning is a misnomer which can refer to a thunderstorm whose precipitation does not reach the ground.

Heat Lightning refers to a lightning flash that is seen from the horizon that does not have accompanying thunder.[79]

Energy

If the quantity of water that is condensed in and subsequently precipitated from a cloud is known, then the total energy of a thunderstorm can be calculated. In a typical thunderstorm, approximately 5×108kg of water vapor are lifted, and the amount of energy released when this condenses is 1015joules. This is on the same order of magnitude of energy released within a tropical cyclone, and more energy than that released during the atomic bomb blast at Hiroshima, Japan in 1945.[10]

Studies

In more contemporary times, thunderstorms have taken on the role of a scientific curiosity. Every spring, storm chasers head to the Great Plains of the United States and the Canadian Prairies to explore the scientific aspects of storms and tornadoes through use of videotaping.[80] Radio pulses produced by cosmic rays are being used to study how electric charges develop within thunderstorms.[81] More organized meteorological projects such as VORTEX2 use an array of sensors, such as the Doppler on Wheels, vehicles with mounted automated weather stations, weather balloons, and unmanned aircraft to investigate thunderstorms which are expected to produce severe weather.[82] Lightning is detected remotely using sensors which detect cloud-to-ground lightning strokes with 95 percent accuracy in detection and within 250 metres (820 ft) of their point of origin.[83]

Mythology

Thunderstorms had a lasting and powerful influence on early civilizations. Romans thought them to be battles waged by Jupiter, who hurled lightning bolts forged by Vulcan. Thunderstorms were associated with the Thunderbirds, held by Native Americans to be a servant of the Great Spirit.[84]Mjolnir, the hammer of the Germanic and Nordic god of thunder, Thor, had the ability to throw lightning bolts. Christian doctrine accepted the ideas of Aristotle's original work, called Meteorologica, that winds were caused by exhalations from the Earth and that fierce storms were the work of God. These ideas were still within the mainstream as late as the 18th century.[85]

Outside of Earth

The clouds of Venus are capable of producing lightning much like the clouds on Earth. The lightning rate is at least half of that on Earth.[86] A thin layer of water clouds appears to underlie the ammonia layer within Jupiter's atmosphere, where thunderstorms evidenced by flashes of lightning have been detected. (Water is a polar molecule that can carry a charge, so it is capable of creating the charge separation needed to produce lightning.)[87] These electrical discharges can be up to a thousand times as powerful as lightning on the Earth.[88] The water clouds can form thunderstorms driven by the heat rising from the interior.[89]

Contents

Cycle

Usually there are more clouds in the afternoon than in early morning. This is because the ground has warmed up enough by the afternoon for strong winds to blow upward from the ground (these winds are called updrafts).[2] If there is very little moisture, an updraft has to go much higher before the water vaporcondenses into a cloud. Sometimes the air can rise up as high as 5,000 to 10,000 feet (1,524 to 3,048 m) before a small cumulus cloud can condense from the invisible water vapor.

High, puffy little clouds do not change until lots of moisture is added. This moisture helps the updraft. Moisture can add heat, which means the cloud will warm up inside and go up even faster. The moisture makes the cumulus cloud "mushroom" upwards and turn into a tall cumuls cloud. The winds inside this cloud are very strong.[2] Thunder and lightning begin when the top of the cloud is 25,000 feet (7,280 m) high. The inside of the cloud is, by this time, cold enough so that the water drops are turned into ice crystals. A cumulus cloud stops growing when it hits the warm stratosphere. Some thunderstorms grow twice as high as Mount Everest!

Strong upper winds in the stratosphere smooth and spread the top of the cloud.[2] This make the cloud look like the top of a mushroom or an anvil. The ice crystals in the anvil cloud give it a fuzzy look. As a thunderstorm grows, water drops or ice crystals inside the cloud hit and mix with each other, getting bigger. The bottom of the cloud grows dark with water about to fall. When the drops become so heavy that the winds inside the cloud cannot keep them up any more, they fall from the cloud as rain or hail.[2] Even as they fall the water drops grow by mixing with the smaller drops, and makes big raindrops.[2] Hail becomes bigger inside the cloud. One thunderstorm cloud usually runs out of energy in about 30 to 50 minutes.

When lightning strikes, energy is let out. This energy moves to the air and makes air spread quickly and send out sound waves. Thunder is the sound that comes from the rapid spread of air along the lightning strike.[1] Thunder is slower than lightning, because light is faster than sound.[2]

Severe thunderstorms

Only about 10% of thunderstorms are thought severe. Severe thunderstorms make things like high winds, hail, flash floods, and tornadoes. Hailstorms damage crops, damage the metal on cars, and break windows. Sudden flash floods that happen because of heavy rains is the biggest reason for weather-related deaths.

Lightning, which happens with all thunderstorms, makes thousands of forest fires each year in the United States. Lightning also kills or injures hundreds of people a year in the United States.

Regular thunderstorms begin when warm air near the ground mix with moist air, which makes an updraft (a wind that goes upward). A severe thunderstorm needs a strong updraft and a strong downdraft. A strong updraft is made when, firstly, the ground is very warm; secondly, when the air is very moist; and thirdly, the air above is very cold. When the updraft gets stronger, so does the thunderstorm.[1]

Flash floods happen when slow-moving thunderstorms pour down much more water than usual in a small area. It rains so hard that the water can not soak into the ground fast enough, and the water rushes down the mountainsides or hills into streams and rivers. These streams and rivers cannot carry all the water, so it quickly floods. The most severe flash flood makes the water level rise dangerously in streams, dry places, or canyons. Flash floods can make terrible mud slides and can move very quickly.[1] They can roll big rocks, tear out trees, and destroy buildings and bridges.

Flash floods can also happen when two or more thunderstorms hit the same spot, one right after the other. They can happen when a dam bursts open or ice breaks up. Flash floods can also happen when it rains very hard on quickly melting snow.

Benefits

Thunderstorms do not do only damage, however: they can be a great help to man and all living creatures.[2] We get lots of water for many continents during the summer. Plants receive lots of life-giving rain when they need it. Without the thunderstorms, many continents would become dry. Fish would die, crops would fail, and animals would perish.[2]

Thunderstorms are also our natural air conditioners. Hot air at the surface rises up into the high atmosphere where it is put out into space. Clouds give us shades, and rain can cool down a hot day.[2] Without thunderstorms, the earth would be as much as 20 F (11 C) warmer.[2] In the summer, dust, haze, and other pollutants come together in the lower atmosphere. When the air rises, either in cumulus clouds or in thunderstorms, spreads the pollution higher up into the atmosphere. Rain from thunderstorms washes away many of these pollutants out of the air.[2]

Lightning in thunderstorms also helps keep the electrical balance between the earth and the atmosphere.[2] Lightning is also fertilizer. When it splits through the sky, it changes nitrogen gas in the air to nitrogen compounds. These fall to the ground and are added to the soil. Nitrogen is one of the main ingredients in fertilizer. Ten percent of the nitrogen fertilizer needed for farming is made by lightning.[2]

So, even though thunderstorms are dangerous, they can be a great help. They give summer water, cool the earth, and clean the air. Lightning balances the earth's electricity and helps fertilize the soil. And after a thunderstorm, sometimes there is a beautiful rainbow.

Keeping safe in a storm

Some tips to keep safe in a thunderstorm are:

If lightning comes near, try to find shelter in a house, car, or low place under some small trees, but not in a shed. Inside a house, do not use the telephone or any electric thing. Do not take a bath or a shower.

If your hair stands on end while outside, immediately drop to the ground and curl up in a ball-like shape.

If there is a flash flood, stay out of low places. Cars should not be driven through water that is flowing over a road or bridge. It might look shallow (not deep), but quickly flowing water can wash a car right off a road or a bridge. It only takes two feet of water to float most cars.[1] Also, the road under the water might already have been washed out, leaving a deep hole. A flash flood can easily turn over a car and trap everyone inside. So, if your car stops in flood waters, get out as fast as possible and run to higher ground. Nearly half the people who die in flash floods in the United States die inside automobiles.[1]